The production of polymers and copolymers of ethylenically unsaturated monomers has gained substantial economic importance. The polymeric products are relatively inexpensive and exhibit a wide variety of commercially desirable properties depending in part upon the nature of the ethylenically unsaturated monomer(s). A number of such polymers are termed thermosetting while others are termed elastomeric and others are thermoplastic. One class of particularly useful polymers is the class of thermoplastic polyolefins resulting from the polymerization of lower .alpha.-olefins such as propylene and ethylene.
In the case of the polymerization of ethylene, the process is relatively uncomplicated in that the product exists in only one steric form. For propylene and other .alpha.-olefins several steric forms exist whose presence depends largely upon the type of catalyst employed. Much if not most of the polypropylene produced commercially is semi-crystalline and in the stereo form termed isotactic. Most recent propylene polymerization processes employ a highly active polymerization catalyst to produce polymer of acceptable properties without the need for extraction to remove amorphous or atactic polymer or deashing to remove catalyst residues. The catalyst residues that do remain in the polymer tend to be acidic and can and do cause problems when the polypropylene polymer is processed. The presence of acidic material may cause polymer degradation when the polymer is processed at elevated temperatures and may also corrode the metal surfaces of polymer processing equipment such as extruders or injection molding equipment. Such corrosion of metal surfaces introduces metallic species into the polymer and increases the likelihood of polymer decomposition or discoloration.
Although the above discussion is primarily in terms of polyolefin polymers, similar considerations apply to other polymeric materials with regard to acid contaminants. For example, the class of linear alternating polymers now known as polyketones is generally produced in the presence of a catalyst composition formed from, in part, acidic materials.
It is known that the adverse effects of acid impurities in polymeric material can be reduced by incorporation of an acid acceptor. For example, U.S. Pat. No. 4,251,407 discloses the use of zinc oxide as an acid acceptor in polypropylene. Other materials known to be useful for this purpose are metal stearates such as calcium stearate and the materials known as hydrotalcites. Although such materials can be helpful when incorporated within polymeric materials, it would be useful to provide additional acid acceptors to reduce the adverse effects of catalyst residues or other acidic impurities in polymeric materials, particularly polyolefins such as polypropylene.